Surface hardened biocompatible metallic medical implants

ABSTRACT

This invention provides surface hardened, abrasion resistant high strength, biocompatible metal medical implants, in particular, titanium alloy implants which do not include any elements which have been shown or suggested as having short term or long term potential adverse effect from a standpoint of biocompatibility. Further, the invention provides methods of strengthening and hardening the surfaces of other titanium, zirconium, and cobalt-based alloy implants with small concentrations of a metal solute such as zirconium, yttrium, tantalum, aluminum, silicon, chromium, or thorium via internal oxidation or nitridation. Alternatively, nitrogen, oxygen, or carbon can be diffused directly into the surface of the implants by interstitial hardening to further increase the surface abrasion resistance of these internally oxidized or internally nitridized implant metal or metal alloys.

This is a division of application Ser. No. 08/120,287 filed Sep. 13,1997, now U.S. Pat. No. 5,415,704 which is a continuation applicationSer. No. 07/832,735 filed Feb. 7, 1992, now abandoned.

BACKGROUND

1. Field of the Invention

This invention relates to surface hardening of biocompatible metallicmetals and alloys, suitable for use as material for a medical implant,including in particular, niobium, titanium, and zirconium based alloyswhich do not include any elements which have been shown or suggested ashaving short term or long term potential adverse biological effects.More specifically, this invention relates to medical implants made ofthese surface-hardened alloys with improved resistance to micro-frettingwear and surface abrasion.

2. Description of the Related Art

The most common materials used for load-bearing medical implants such asorthopedic or cardiovascular implants, are metallic alloys, ceramics,and composites formed from biocompatible polymers and variousreinforcing materials.

Metals and metal alloys such as stainless steel, Co-Cr-Mo alloy,titanium, and titanium alloys have been used successfully for many yearsas implant materials, particularly for orthopedic applications. Thesematerials have the requisite strength characteristics needed for suchimplants but are susceptible to fretting, wear, and corrosion in thebody unless treated to reduce these effects. Particulates produced byjoint articulation processes or micromotion between assembled devicestend to cause accelerated wear of prosthetic joints and trauma devices.

Further, concern has been raised about potential abrasion betweenimplant metals and adjacent bone and bone cement. This abrasion createsparticulates which are associated with adverse cellular response,including bone cell death and eventual loosening of the implant andsubsequent revision.

To prevent micro-fretting of the implant surface, the surface may becoated with an amorphous diamond-like carbon coating or a ceramic-likecoating, such as titanium nitride or titanium carbide, using chemical orplasma vapor deposition techniques to provide a hard, impervious, smoothsurface coating. These coatings are especially useful if the prosthesisis subjected to conditions of wear, such as, for instance, in the caseof bearing surfaces of knee or hip prostheses, or between screws andbone plates or modular implants. For the case of orthopedic implantbearing surfaces, bone cement fragments can abrade these relatively thin(up to about 5 microns) surface coatings with time and create hard,abrasion products from these coatings which in turn further accelerateabrasion and wear of the softer underlying metal substrate.

Methods for providing amorphous diamond-like carbon coatings are knownin the art and are disclosed in, for example, EPO patent application 302717 A1 to Ion Tech and Chemical Abstract 43655P, Vol. 101 describingJapan Kokai 59/851 to Sumitomo Electric. Chemical Abstract 43655P,Volume 101 describes the Sumitomo Electric patent as: "Prosthetics madeof Ti, A1, and stellites are coated with a film of diamond ordiamond-like C to increase the resistance against wear. Thus, anartificial bone joint was prepd. using pure Ti. This was coated with 1μ-thick diamond-like C, using C₂ H₆ in induction heating. The productwas biocompatible and resistant to wear in the rabbit." EPO patentapplication 302 717 A1 describes methods of producing hard diamond-likecoatings by direct beam deposition "from a saddle field source fromhydrocarbon precursor gases such as propane, butane, and acetylene." EPOpatent application 302 717 A1 also notes that "The hard carbon coatingcan be produced . . . . by chemical vapour depositions . . ." and "Acoating thickness of 500 Angstroms is preferred although thicknesses upto 2000 Angstroms may be provided for increased protection."

With orthopaedic and cardiovascular implants being implanted in youngerpeople and remaining in the human body for longer periods of time, thereis a need for an implant material with requisite strength and highabrasion resistance which minimizes the production of abrasive particlesfrom surface abrasion effects.

SUMMARY OF THE INVENTION

The invention provides novel hardened metallic implants with highabrasion resistance and methods for increasing implant abrasionresistance without the use of a hard deposited overlay ceramic-likecoating. In one embodiment of the invention implants internal oxidationor nitridation is used to harden the implants' surfaces. While a minimalexternal surface oxide or nitride scale may form on these hardenedimplants, surface strengthening is primarily due to dispersionstrengthening by internal oxidation or nitridization just below theimplant's surface. Thus, there is no significant hard external scaleformed which may spall off or produce abrasion products to damage theimplant surface or cause undesirable biological effects.

In another embodiment, the invention also provides for the additionalhardening of the implant surface with interstitial diffusion of oxygen,nitrogen, or carbon. This latter method is used on implants which havepreviously been treated with an internal oxidation or nitridizationtreatment. A lower diffusion hardening temperature can be utilized whichwill not affect the previously internally oxidized or nitrided benefits.

The invention oxidizing or nitriding method of surface hardening implantmetals provides a fine oxide or nitride dispersion within the metalsurface without a significant surface scale so that the implantessentially retains its metallic appearance.

Standard nitriding, oxidizing, and carbonizing treatments for metals areavailable and known to persons skilled in the art. These methods useplasma, fluidized beds, molten salts, or nitrogen, oxygen, orcarbon-containing gaseous environments at elevated temperatures toperform surface treatments. In these methods, diffusion of nitrogen,carbon, and oxygen into the metallic implant and the subsurfacenucleation of nitrides, carbides, or oxides increases hardness andstrengthens the metal to depths of 50 microns or more depending on gasconcentration, time, temperature, and alloy composition. However, theformation of nitrides or carbides in chromium-containing metals occursvia formation with the chromium. Thus, local depletion of chromium canoccur in the matrix adjacent to the nitride or carbide particle andreduce corrosion resistance. Oxygen diffusion hardening of titaniumalloys promotes an undesirable, weaker alpha case on the surface.Internal oxidation or nitridization avoids introducing these limitationsof corrosion resistance and loss of strength.

While surface hardening by conventional oxygen, nitrogen, or carbondiffusion forms no significant external nitride or carbide scale, ifdone at sufficiently low partial pressures of these diffusing species,the metallic implant's surface hardness can exceed 50 Rockwell C in thecase of certain titanium alloys, significantly higher than for theuntreated alloy, which can vary up to about 40 Rockwell C depending onthe metal and the heat-treated condition of the metal. Further,conventional diffusion hardening surface treatments can significantlyimprove surface abrasion and fretting resistance for titanium alloys andstainless steels. Surface hardening by diffusion may be more effectivewith some metals than others, depending on the rate of diffusion in theparticular metal, and the stability of the microstructure at thediffusion hardening temperature. Further, diffusing species such asnitrogen or carbon may react with chromium in cobalt alloys or stainlesssteel, rendering them less corrosion resistant. Internal oxidation caneliminate the latter concern.

The internal oxidation or nitridization process is applicable to almostany metallic implant. To produce the invention hardened implant, a lowconcentration of a more readily oxidizable (or nitridable) metal soluteis added to the metallic implant's alloy composition, so that theinternal oxidation (or nitridization) process strengthens and hardensthe surface via reaction of this specific solute with diffusing oxygen(or nitrogen). This is in contrast to conventional diffusion hardeningmethods described above. Selection of a suitable metal solute forinternal oxidation or nitridization is based upon the thermodynamics ofthe process. Specifically, the more negative the value is for the freeenergy of formation (i.e., ΔG°) for a particular metal oxide (e.g. Ta₂O₅), the greater the tendency (i.e. thermodynamic driving force) to formthe oxide at a given temperature within the metal or alloy.

For internal oxidation surface hardening and strengthening, the depth ofthe internally oxidized zone, the oxide particle size, and the strengthof the alloy depend on the oxidizable solute's concentration, theconcentration of oxygen in the oxidizing environment, the oxidationtemperature, and the oxidation time. Because the strength of the surfaceis increased, the implant's fatigue strength in tension bending willalso increase, as well as its surface hardness and subsequent abrasionresistance.

While the invention surface hardened implants possess a relatively highstrength, the usefulness of these hardening processes is not limited totreating implants in load-bearing applications. Because of its abrasionresistance, enhanced fatigue strength, and tension bending strength, thehardened implants can be used in many types of implants including, butnot limited to, hip joints, knee joints, compression hip screws, toothimplants, skull plates, fracture plates, intramedullary rods, staples,bone screws, cardiovascular implants, such as heart valves andartificial heart and ventricular assist devices, and other implants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a surface hardness vs. depth curve (from the surface) for aCo-Cr-Mo alloy coupon alloyed with 1 wt. % tantalum which has beensurface hardened by internal oxidation at a temperature of 500° C. for20 hours.

FIG. 2 is a surface hardness vs. depth curve for a Ti-13Nb-13Zr alloycoupon alloyed with 1 wt. % tantalum which has been surface hardened byinternal oxidation at a temperature of 700° C. for 20 hours.

FIG. 3 is a surface hardness vs. depth curve for a Ti-6A1-4V alloycoupon alloyed with 1 wt. % tantalum which has been surface hardened byinternal oxidation at a temperature of 800° C. for 20 hours.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. The Implant Metals

The invention provides surface hardened, abrasion resistant medicalimplants. The hardening methods may be applied to all currently usedimplant metals include AISI 316L stainless steel (i.e. Fe-Cr-Ni-Mo),Co-Cr-Mo (F75 or F799) and Ti-6A1-4V, and those alloys that may becontemplated as useful. While the methods for producing the inventionabrasion resistant metallic implants may be applied to almost any metalor alloy used to fabricate implants, the Co-Cr-Mo and titanium alloysare preferred in the present invention. Consequently, much of thedescription will discuss titanium alloy implants, it being understoodthat other alloys may also be similarly processed by the inventionmethods to produce the invention surface hardened implants. Examples ofpreferred titanium alloys include Ti-6A1-4V, beta titanium alloys,Ti-Mo, Ti-A1Nb, TiV and Ti-Fe. Newer, more biocompatible titanium alloycompositions with lower elastic moduli such as Ti-Zr, Ti-Nb-Zr andTi-Nb, are also capable of internal oxidation, as are zirconium andniobium based alloys. Examples of such compositions may be found in ourapplication U.S. Ser. No. 07/454,181, filed Jan. 28, 1991, which isfully incorporated by reference. Other metals and metal alloys that maybe employed include, but are not limited to, zirconium and zirconiumalloys such as Zr-Nb, for example, and stainless steels, such as, forexample, stainless steel 316L (Fe-Cr-Ni-Mo).

The most preferred biocompatible, low-modulus titanium alloy implantsmay be produced by combining, as commercially pure components, titanium,niobium, and zirconium in the appropriate proportions. The titaniumalloys (depending on the alloying constituents) may further containconcentrations of another metal (typically less than 2 wt. %), such astantalum, yttrium, thorium, chromium, aluminum, and the like, forpurposes of internal oxidation. Ti-Nb alloys may contain zirconium,aluminum, yttrium, hafnium, or tantalum in concentrations less thanabout 2 wt. % for the same purpose. Zirconium or zirconium alloys maycontain less than about 2 wt. % tantalum, yttrium, thorium, or aluminumfor this purpose. Cobalt alloys and stainless steels may contain lessthen about 2 wt. tantalum, aluminum, and yttrium for this purpose.

One preferred alloy contains titanium as the major component comprisingabout 74 wt. % of the alloy in combination with about 13 wt. % ofzirconium and 13 wt. % niobium. Less than about 2 wt. % of another metalsuch as tantalum or aluminum is added to replace the same amount oftitanium, or zirconium, or to a lesser degree, niobium (since niobiumconcentration is important to establish a low modulus). The other groupof preferred alloys also contains titanium as the major component,comprising about 84 to 87 wt. % of the alloy, in combination with about13-17 wt. % niobium, and less than about 2 wt. % tantalum, yttrium,hafnium, thorium, aluminum, and chromium. The additional metal addedpreferably replaces the same amount of titanium or to a lesser degreeniobium. The most preferred biocompatible low modulus alloys includeTi-13Nb-13Zr and 1% Ta; and, Ti-15Nb with 1% Ta.

The methods of making and using implants of titanium, niobium (andoptionally tantalum as a substitute for niobium) and optionallyzirconium or zirconium alloy (and optionally tantalum, yttrium, oraluminum as a substitute for zirconium) is described in our copendingapplication U.S. Ser. No. 07/454,181, filed Jan. 28, 1991, referred toabove. The machining, casting or forging of the alloys into the desiredimplant shape may be carried out by any of conventional methods used fortitanium or zirconium alloys.

Implants fabricated from the titanium alloys may be supplied with asintered porous bead or wire or plasma-sprayed coating of titanium alloyof the same or different composition, or even pure titanium, to allowstabilization of the implant in the skeletal structure of the patientafter implantation by bone ingrowth into the porous structure.Similarly, zirconium or zirconium alloys may be supplied with thesimilar porous coatings of zirconium or zirconium alloy. Such porousstructures are occasionally attached to the implant surface bysintering. Similarly, cobalt alloys may be supplied with a porous metalcoating. Because sintering of porous coating is preferred attemperatures above 1000° C., the mechanical properties of titaniumalloys can change significantly due to substantial grain growth, phasechanges, and other metallurgical factors arising from the sinteringprocess. Thus, for example, after sintering to attach the porouscoating, additional heat treatments are desired to restore mechanicalproperties. To avoid these potential restrictions, the implant metal oralloy may be porous coated using plasma spray methods in which minimalheating occurs to the base material.

While the preferred titanium alloys themselves provide a non-toxicmaterial suitable for orthopedic, cardiovascular, or other medicalimplants, it is desirable, for other reasons, such as micro-frettingagainst bone or polyethylene bearing surfaces, or microfretting, toharden the surface of an implant fabricated from these alloys.Furthermore, in the case of non-titanium alloys, such as the stainlesssteels or cobalt alloys, surface hardening may be desirable for bothincreasing fatigue strength and the reduction of micro-fretting orabrasive wear.

II. Hardening and Strengthening the Alloy Surface

The invention provides methods to strengthen and harden an alloy'ssurface and includes (1) an internal oxidation process which includesadding a low concentration of a more readily oxidizable solute such asyttrium, niobium, tantalum, zirconium, thorium, hafnium, chromium, oraluminum to the alloy, and oxidizing a portion of the solute that isfound immediately beneath the metal surface; (2) an internalnitridization process which includes adding low levels of a relativelymore nitridable solute such as zirconium, silicon or thorium to thealloy and nitriding a portion of this solute that is found just belowthe metal surface; and (3) an additional interstitial diffusionstrengthening process using nitrogen, oxygen, or carbon. For theinternal oxidation and nitridization processes, the particularoxidizable or nitridable solute selected will vary, depending upon theparticular metal or metal alloy composition of the implant. Each ofthese three methods will be discussed in turn.

Internal Oxidation

In the internal oxidation process, a small quantity of a "more readilyoxidizable solute" is added to the alloy from which the implant is to befabricated. By more readily oxidizable solute, we mean one that oxidizesmore readily than the other constituent metals of the alloy with whichthe solute is admixed or alloyed. During controlled oxidation, oxygendiffuses into the implant's surface leading to the formation of a finedispersion of solute oxide particles on and immediately below the alloysurface. The depth of this internally oxidized zone, the oxide particlesize, and hardening and substrate strengthening of the surface dependson the solute concentration, the concentration of oxygen in theoxidizing environment, the oxidation temperature, and the oxidationtime. Because both the hardness and strength at the surface isincreased, the fatigue strength will also increase. For example,Rowcliffe, et al. (Oxide Dispersion Strengthening, Ansell et al., ed.,Gordon and Breach, N.Y., "Strengthening of Niobium-Zirconium Alloys byInternal Oxidation," 1968) have shown that the yield and ultimatetensile strength of Nb-1Zr alloy can double by internal oxidation. Theirresearch was, however, aimed at improving high temperature creepresistance of the alloy and there is no teaching or suggestion of theuse of such a method to produce a surface-hardened medical implant forimproved wear and abrasion resistance.

In the case of titanium or Ti-Nb alloys, tantalum may be added as asolute, because tantalum is more reactive with oxygen (lower free energyof formation) than either titanium or niobium. Thus, it is possible tointernally oxidize the alloy without oxidizing niobium or titanium ifthe concentration of oxygen (partial pressure of oxygen) is less thanthat required to form niobium or titanium oxides. Similarly, forTi-Nb-Zr or Zr-Nb alloys with tantalum added as a solute, theconcentration of oxygen is less than that required to form titanium,niobium, or zirconium oxides. Additionally, when tantalum is added as asolute, to Co-Cr-Mo alloys, then the more oxidizable tantalum causesinternal oxidation of the alloy. Table 1 is a list of published standardfree energies of formation (ΔG°) for various titanium, niobium,zirconium, tantalum, aluminum, yttrium, and other oxides. The morenegative the ΔG° value, the greater the tendency (i.e. thermodynamicdriving force) to form the oxide at a given temperature, T(°K).

                  TABLE 1                                                         ______________________________________                                        ΔG° = A + BT.sub.log T + CT                                                           -ΔG (K cal.)                                       Oxide   A         B      C       600°                                                                         800°                            ______________________________________                                        Al.sub.2 O.sub.3                                                                      -405      4      -92     475   495                                    Cr.sub.2 O.sub.3                                                                      -268      1      -62     323   333                                    CoO     --        --     --      261   275                                    HfO.sub.2                                                                             -268      2      -78     334   350                                    MoO.sub.2                                                                             -141      5      -56     181   192                                    NiO      -58      --     -24      80    84                                    NbO     --        --     --      119   130                                    SiO.sub.2                                                                             -215      --     -42     253   261                                    Ta.sub.2 O.sub.5                                                                      -491      31     -168    644   666                                    ThO.sub.2                                                                             -293      2      -48     332   342                                    TiO.sub.2                                                                             --        --     --      280   240                                    TiO     -122      1      -21     141   144                                    VO      --        --     --      137   145                                    Y.sub.2 O.sub.3                                                                       -420      --     -66     479   493                                    ZrO.sub.2                                                                             -260      5      -60     301   313                                    ______________________________________                                    

Thus, those skilled in the art can appreciate that the low-modulustitanium-niobium and titanium-niobium-zirconium alloys described in U.S.Ser. No. 07/454,181 and other titanium, cobalt, and zirconium alloysdescribed above can be internally oxidized by the presence of smallquantities of relatively more oxidizable solutes such as yttrium,chromium, aluminum, hafnium, and tantalum. Tantalum and aluminum can beused for internally oxidizing Ti-Nb-Zr alloys as their ΔG° is morenegative than even that of zirconium or niobium. However, based onbiological factors, aluminum is not preferred over tantalum. Thesebiological factors are partially mitigated because the aluminum near thesurface of the implant converts to inert aluminum oxide so that thepresence of aluminum will not be as potentially detrimental as ifaluminum metal were present in the bulk alloy. The alloys that may beinternally oxidized via the presence in the alloy of small amounts (inparentheses) of zirconium, tantalum, or aluminum include, but are notlimited to, Ti-Nb-Zr-(Ta), Ti-Nb-(Zr), Ti-Nb-(Ta), and Ti-Nb-(Cr),Ti-Al-V-(Ta), Co-Cr-Mo-(Ta), and 316L stainless steel (Ta). Othercombinations are also possible based on relative differences in ΔG°between the preferred solute and other alloy constituents.

In the internal oxidation process, the depth of internal oxidation (flatplate surface) from the surface (X) for a given period of time (t) isrepresented by the following equation.

    X=[2N.sub.0.sup.(s) D.sub.0 t/vN.sub.b.sup.(0) ].sup.1/2

where: No.sup.(s) is the concentration of oxygen at the surface (lessthan that required to oxidize either the titanium or niobium base alloy,for example if tantalum is added as the solute); D_(o) is the rate ofdiffusion of oxygen into the surface; v is the stoichiometric value(ratio) of oxidizing solute species to oxygen (i.e., v=2 for ZrO₂); andNb(0) is the concentration of the solute which is being internallyoxidized (i.e., chromium, aluminum, yttrium, or tantalum, and the like).

For internal oxidation, the depth increases with decreasingconcentration of solute. The value N_(o).sup.(s) is related, by areaction rate constant, to the square root of the partial pressure ofoxygen.

To optimize the strengthening effect of the internal oxidation process,the advancing front of diffusing oxygen should progress rapidly. Thisresults in more nucleation and less growth of internal oxide particles,and therefore smaller, more effective particles. Relatively loweroxidation temperatures are also preferred to minimize particle growth,but presents a trade-off with the rate of oxygen diffusion. Thus, thepartial pressure of oxygen and the solute concentration should bemaximized (preferably less than about 2 wt. %). Various times andtemperatures are possible and can be optimized experimentally.

To internally oxidize a particular alloy implant containing a morereadily oxidizable added solute such as tantalum, yttrium, aluminum, orzirconium, the alloy implant is packed in a mixture of a metal and itsoxide whereby the metal selected for the packing mixture is the nextmost readily oxidizable solute compared to the added metal solute (e.g.tantalum and zirconium). Oxygen is evacuated from the implant'senvironment and residual oxygen is scavenged by, for example, resistanceheating a zirconium or tantalum wire in the evacuated environment. [Thisis called "getting" the oxygen to a sufficiently initial low level.].Upon heating the alloy implant, the metal oxide in the packing and themetal in the packing attain an equilibrium which produces the requiredlow level of oxygen concentration (partial pressure) in the system,sufficient to react primarily with the low concentration of more readilyoxidizable metal solute present in the implant and to thereby internallyoxidize the metal implant's surface.

For example, to internally oxidize a Ti-6A1-4V implant with a tantalumsolute, the implant is evacuated in a packed mixture of aluminum andaluminum oxide, since aluminum is the most readily oxidizable metal nextto tantalum. Any remaining oxygen in the evacuated system is reduced byresistance heating a zirconium or tantalum wire in the evacuatedenvironment. Upon heating to about 500° C.-1200° C. for up to about 100hours (depending upon the desired depth and hardening), the aluminummetal and aluminum oxide establish an oxygen partial pressure (orNo.sup.(s)) sufficient to produce oxidation of the tantalum solute bydiffusion of oxygen into the alloy surface, but without causing thetitanium, aluminum, or vanadium in the alloy to form a significantexternal oxide scale. For this case, it is more desirable to establishthe partial pressure of oxygen close to 10⁻⁶ torr because levels near10⁻⁵ torr tend to produce aluminum monoxide (AlO) near the surface. Atoxygen pressures less than 10⁻⁶ torr, the strengthening effect is lesspronounced (i.e. more growth vs. nucleation of the internal oxideparticles) due to lower levels of oxygen and slower penetration(diffusion) rates of oxygen into the surface. Oxidation depth shouldpreferably be less than about 200 microns, most preferably less thatabout 100 microns in order to minimize particle size (less than about150 angstroms), processing time, and temperature, and to optimizehardness and strength.

The preferred internal surface strengthened metallic implants havingexcellent biocompatibility include those produced by the internaloxidation of shaped Ti-Nb alloys using small concentrations ofzirconium, yttrium, or tantalum (i.e. Ti-Nb-Zr, Ti-Nb-Y, or Ti-Nb-Ta),or internal oxidation of Ti-Nb-Zr implants or zirconium or zirconiumimplants using small concentrations (less than about 2 wt. %) oftantalum (i.e. Ti-Nb-Zr-Ta or Zr-Ta) or yttrium. Additionally, aluminumsolute levels less than about 2 wt. % would be effective for internallyoxidizing these implants.

Internal Nitridization

As an alternative to internal oxidation, internal nitridization can beperformed by adding low levels (less than about 2 wt. %) of a solutemore nitridable than the base alloy constituents. By a "more nitridablesolute," we mean one that nitrides more readily than the otherconstituent metals of the alloy with which it is admixed or alloyed. Inthe case of the titanium alloys, solutes such as zirconium, silicon, orthorium may be added. The titanium alloys (with solutes for internalnitridization in parentheses) suitable for internal nitridization viathe presence of small amounts (i.e. less than 2 wt. %) of silicon,thorium, or zirconium include, but are not limited to, Ti-Nb-(Si),Ti-Nb-(Th), and Ti-Nb-(Zr). Zirconium alloys should contain less thatabout 2 wt. % of silicon or thorium, and cobalt alloys and 316Lstainless steel should contain less than about 2 wt. % of thorium,tantalum, aluminum, silicon or zirconium. Table 2 is a list of thepublished standard free energies of formation (ΔG° ) for the nitrides ofthese elements:

                  TABLE 2                                                         ______________________________________                                        ΔG°, Kcal, = A + BTlogT + CT                                     Nitride                                                                              A (K Cal)  B       C        -ΔG at 800° C.                ______________________________________                                        TiN    -160           --    -45        210                                    NbN    -114    (est)  --    -50   (est)                                                                              169                                    ZrN    -175           --    -46        226                                    TaN    -117           14    -80        170                                    Cr.sub.2 N                                                                           -52            11    -66        105                                    AlN    -154           --    -44        202                                    Si.sub.3 N.sub.4                                                                     -177           6     -97        264                                    Th.sub.3 N.sub.4                                                                     -310           --    -90        409                                    Mo.sub.2 N                                                                           -34            9.2   -58         80                                    ______________________________________                                    

One skilled in the art can see that unlike the free energy of formationof the oxide (see Table 1), tantalum, chromium, and aluminum would nottend to effectively form internal nitride particles as compared to thetitanium or niobium base metal alloy which have similar free energy offormation values. For example, even zirconium has only a slightly morenegative ΔG° than titanium or niobium. Thus, for internal nitridizationof a Ti-Nb low modulus alloy, small concentrations (less than about 2wt. %) of either silicon or thorium are preferred because of theirsignificantly more negative levels of free energy of formation. A smallconcentration of zirconium is also effective, but to a lesser degree.

Similar internal nitridization relationships apply as for internaloxidation. However, the packing material used to establish theappropriate concentration (partial pressure) of nitrogen would be acombination of the metal and metal nitride of the most nitridable (nextto that of the solute) metal constituent in the alloy.

It should be noted that for both internal nitridation and oxidation, asubstantial portion of the respective solute added that occurs at theimplant's surface will undergo nitridization or oxidation. Soluteconcentrations are preferably less than about 2 wt. %. While more solutemay be added, surface hardening will not be as effective therefore suchaddition is not preferred.

Interstitial Diffusion Strengthening

Another method which may be used to improve the surface abrasionresistance of metallic implants, especially the preferred titaniumimplants such as Ti-Nb-Zr, Ti-Nb, Ti-Nb-Zr-Ta, Ti-Nb-Ta and zirconiumand zirconium alloys, is "interstitial diffusion" strengthening of thesurface with oxygen, nitrogen, or carbon. Treatments for interstitialdiffusion strengthening of metals generally use gaseous or liquidnitrogen or carbon-containing environments or gaseous environments,including fluidized beds, or similar oxygen-containing environments, atelevated temperatures, and are known to those skilled in the art.Diffusion strengthening to depths of less than 100 microns are usuallysufficient. In the interstitial diffusion strengthening process, oxygen,nitrogen, and carbon concentrations are kept sufficiently low so thatthere is no significant formation of an external oxide nitride orcarbide scale so that an essentially metallic-type appearance remains onthe surface. The surface hardness of a typical 33 Rockwell C hardnesstitanium alloy when so treated can exceed 60 Rockwell C hardness and canalso significantly improve surface abrasion and fretting resistance.

While the general interstitial diffusion hardening process using oxygen,nitrogen, and carbon is used commercially and is well known to those ofskill in the art, its application to Ti-Nb, Ti-Nb-Zr, zirconium,Ti-6A1-4V, Co-Cr-Mo, and other medical implant metals capable ofinternal oxidation or internal nitridization is novel. This diffusionhardening process can be applied to the preferred titanium alloy,zirconium alloy, cobalt alloy, and stainless steel compositions, whetheror not these alloy compositions have received a previous internaloxidation or internal nitridization treatment, to achieve additionalbenefits.

It should be noted that the internal oxidation and interstitialdiffusion hardening methods can be controlled to minimize or eliminatethe formation of an external scale-type coating. Therefore, the hardenedimplant's surface may, with little or no additional surface treatment,be coated with, for example, amorphous diamond-like carbon, pyrolyticcarbon, TiN, TiC, and oxide coatings and the like, which are physical orchemical vapor deposited, for further improved abrasion resistance, ifdesired. The hardening treatment of any of the methods described in thisspecification would improve the attachment strength of these coatings.

The following examples are intended to illustrate the invention asdescribed above and claimed hereafter and are not intended to limit thescope of the invention in any way.

EXAMPLE 1

A coupon formed of Co-Cr-Mo alloyed with about 1 wt. % of tantalum wasplaced in a tube and packed with chromium and chromium oxide. The tubewas evacuated to remove essentially all oxygen and sealed. The tube wasthen placed in a furnace and heated to a temperature in the range 500°to 800° C. for a certain time period as indicated in Table 1 below. Thetube was then allowed to cool to room temperature. This procedure wasalso applied to a Ti-13Nb-13Zr alloy (packed with zirconium and ZrO₂)and a Ti-6A1-4V alloy (packed with aluminum and Al₂ O₃), each containingabout 1 wt. % tantalum, both of which were heated to the temperaturesand for the time periods indicated in Table 3 below. The bulk hardness,surface hardness, and hardness gradient depths for each of the threealloys after internal oxidation at the indicated conditions are shown inTable 3.

                  TABLE 3                                                         ______________________________________                                        Summary of Internal Oxidation Surface Hardening                               Results For Selected Implant Materials                                                Internal Oxida-             Hardness                                          tion Parameters                                                                         Bulk     Surface  Gradient                                            Temp    Time    Hardness                                                                             Hardness                                                                             Depth                                 Material  (C.°)                                                                          (Hrs.)  (Knoop)                                                                              (Knoop)                                                                              (Micron)                              ______________________________________                                        Co--Cr--Mo                                                                              500     1       300    350     50                                   (F-75)    500     6       340    550    150                                   w/1 wt. % Ta                                                                             500*   20      340    600    200                                             600     1       310    650    200                                             600     6       300    500    250                                             600     20      310    650    300                                             700     1       300    600    200                                             700     20      340    600    250                                             800     1       280    450    100                                             800     6       350    500    200                                   Ti-13Nb-13Zr                                                                             700*   20      225    400    150                                   w/1 wt. % Ta                                                                            700     100     225    425    200                                             800     1       215    300    250                                             800     6       240    350    >300                                  Ti-6Al-4V 800     1       275    375    100                                   w/1 wt. % Ta                                                                             800*   20      280    375    150                                   ______________________________________                                    

FIGS. 1-3 represent the surface hardness vs. depth curves for the alloymaterials marked with asterisks as shown in Table 3. Specifically, FIG.1 is the surface hardness vs. depth curve for a Co-Cr-Mo alloycontaining 1 wt. % Ta which has been heated to 500° C. for 20 hours.FIG. 2 represents the surface hardness vs. depth curve for aTi-13Nb-13Zr alloy containing 1 wt. % Ta which has been heated to 700°C. for 20 hours. FIG. 3 represents the surface hardness vs. depth curvefor a Ti-6A1-4v alloy containing 1 wt. % Ta which has been heated to800° C. for 20 hours. The peak hardness values given in Table 3 are notnecessarily as great as that produced by the curves shown in FIGS. 1-3.Extrapolation of these curves suggest a still higher hardness at theimmediate surface.

While it is clear that a chemical element of the Periodic Table that isa metal is a pure metal, alloys of metallic elements are also referredto as "metals," in the sense that they are metallic rather than ceramicorganic, etc. Therefore, in the specification and claims when referenceis made to a "metal implant," this includes an implant fabricated from ametal alloy as opposed to a ceramic or polymer or composite. Further, inthe specification and claims when an alloy is combined with a metalsolute, it forms a "solute-containing alloy." Thus, when an alloy of agiven composition, commercially available or specially made, isspecified, its composition does not include the metal solute, unless sospecified. Therefore, in the claims the metal implant's composition isspecified in terms of (1) a metal composition and (b) the metal solutesubsequently added to then make the solute-containing alloy from whichthe implant body is fabricated.

The invention has been described with reference to its preferredembodiments. After studying this disclosure, a person of skill in theart may appreciate changes and modifications that are within the spiritof the invention as disclosed above and within the scope of the claimsherebelow.

I claim:
 1. A surface hardened metallic medical implant, the implantcomprising:a surface hardened metallic alloy implant body with surfacehardness greater than about 40 Rockwell C, the hardened surface producedby a process comprising: (i) selecting a metallic alloy containing lessthan about 2 wt. % of a metal solute different from and more readilynitridable than other elements of the metallic alloy implant body; (ii)hardening the surface of the implant body by internal nitridation of asubstantial proportion of the more readily nitridable solute at thesurface of the implant while maintaining a metallic luster on thesurface of the implant.
 2. The surface hardened metallic medical implantof claim 1, wherein said implant body comprises about 85 wt. % titaniumand about 15 wt. % niobium; and the metal solute is selected from thegroup consisting of silicon, tantalum, and thorium.
 3. The surfacehardened metallic medical implant of claim 1, wherein the implant bodycomprises about 73 wt. % titanium, about 13 wt. % niobium, and about 13wt. % zirconium; and the metal solute is selected from the groupconsisting of silicon and thorium.
 4. The surface hardened metallicmedical implant of claim 1, wherein the implant body comprises Co-Cr-Moalloy and the metal solute is selected from the group consisting ofsilicon, tantalum and thorium.
 5. The surface hardened metallic medicalimplant of claim 1, wherein the implant body comprises Ti-6A1-4V; andthe metal solute is selected from the group consisting of silicon,tantalum, and thorium.
 6. The surface hardened metallic medical implantof claim 1, wherein the metallic implant body is further hardened bydiffusion into the metallic alloy of an element selected from the groupconsisting of nitrogen, oxygen, and carbon.